Lifestyles change and the demand from restaurants and catering firms for colourful and interesting garnish and even from the housewife for ready-to-use processed salads continue to rise. As a result, the breeding companies are looking for varieties with prominent colour, better taste and a wide variety of texture. The lettuce market can be divided into three groups, namely entire whole heads, pre-cut whole heads and baby leaves.
At present the attractive red colour in pre-cut lettuce mixtures is often provided by the presently available “red” lettuce (Lactuca sativa), or by radicchio rosso (Cichorium intybus), red cabbage or red chard. The “red” lettuce that has been known so far is not really red throughout the leaves. In particular the head forming types are either only red along the leaf edges or speckled with red. They are never completely red in the heart. This is caused by the fact that the red colour is only expressed in those parts of the head that are exposed to daylight, more specifically to UV-radiation. Leaves of the so-called “red” lettuces are therefore for the main part green and their red does hardly contribute to the red colour impression of a pre-cut lettuce mixture.
Often the transitional stage between the red part and the green part of the leaf is brown-coloured. This brown colour is considered as visually unattractive. Red speckled lettuces are associated with plant diseases or bloodstains. At present plant breeders select against the brown colour as well as against the speckled red colouring. In addition, it is even found that speckled or brown coloured lettuce leaves are often discarded, both in the lettuce packaging industry and by the consumer.
The disadvantage of using leaves from other vegetables than lettuce is that the different taste of these other vegetables is often experienced as undesirable. Radicchio rosso is for example a Cichorium intybus that has a bitter taste. The texture of red cabbage is entirely different from the much softer texture of lettuce.
Anthocyanin synthesis in lettuce is induced by UV-radiation (see for example Voipio & Autio, 1995, Responses of red-leaved lettuce to light intensity, UV-A radiation and root zone temperature. In: Acta Horticulturae 399. Greenhouse environmental control and automation. Eds. B J Bailey, T Takakura. Kyoto, Japan. p 183-187; Benoit, et al., 1998, Effect of a photoselective greenhouse film on a few vegetable crops in the Belgian North Sea climate. In: 14th International congress on plastics in agriculture, Tel Aviv, Israel, March 1997. Laser Pages Publishing, Jerusalem, Israel. p 81-92; Krizek et al., 1998, Inhibitory effects of ambient levels of solar UV-A and UV-B radiation on growth of cv. New Red Fire lettuce. Physiologia Plantarum 103(1), p 1-7; Kleinhenz et al., 2003, Variety, shading, and growth stage effects on pigment concentrations in lettuce grown under contrasting temperature regimens. Hortechnology 13(4), p 677-683). The development of a red colour in many plant species is dependent on the production of anthocyanin.
The many different varieties of lettuce are often grouped into three types. The most common is head lettuce of which there are the crisp head (or iceberg) and butter head. Romaine lettuce (or cos lettuce) forms a loose upright head. The “leaf” lettuce types are non-heading and loose leafed.
In head-forming lettuce types the heart of the head is to a more or lesser extent closed and cannot be reached by light. It is thus not possible to have anthocyanin production in closed heads such as iceberg lettuce and butter lettuce or in romaine lettuce, which has a loosely closed head.
A related problem is the lack of red colouration of red lettuce grown in glasshouses, plastic tunnels, or in closed containers. Due to the lack of UV-radiation, which is reflected by the glass or plastic covering or which is lacking in the artificial assimilation light spectrum, the expression of anthocyanin is much less than in outdoor conditions. This reduces the possibilities to produce red lettuce under indoor conditions, and therefore the possibility to produce red lettuce protected from adverse weather conditions, like cold, heat, and all sorts of precipitation.